An electric-power-steering controller is described below as a conventional vehicle controller. FIG. 4 is a circuit diagram showing the conventional electric-power-steering controller disclosed in, for example, Japanese Patent Application No. 5-64268, in which the controller is locally shown by a block diagram. In FIG. 4, symbol 40 denotes a motor for outputting an auxiliary torque to the steering wheel (not illustrated) of a vehicle and 41 denotes a battery for supplying a motor current IM for driving the motor 41.
Symbol 42 denotes a large-capacity (1,000 to 3,600 .mu.F.) for absorbing the ripple component of the motor current IM, 43 denotes a shunt resistor for detecting the motor current IM, and 44 denotes a bridge circuit comprising a plurality of semiconductor switching devices (e.g. FETs) Q1 to Q4 for switching the motor current IM in accordance with the magnitude and direction of the auxiliary torque. Symbol 46 denotes a normally-open relay for supplying or cutting off the motor current IM according to necessity.
Symbol 47 denotes a driving circuit for driving the motor 40 through the bridge circuit 44 and moreover driving the relay 46 and 48 denotes motor-current detection means for detecting the motor current IM through an end of the shunt resistor 43. The driving circuit 47 and motor-current detection means 48 constitute the peripheral circuit element of a microcomputer to be described later.
Symbol 50 denotes a torque sensor for detecting the steering torque T of a steering wheel and 51 denotes a speed sensor for detecting the speed V of a vehicle.
Symbol 55 denotes a microcomputer (ECU) for computing the auxiliary torque in accordance with the steering torque T and vehicle speed V and moreover, generating a driving signal corresponding to the auxiliary torque by returning the motor current IM, which inputs a rotational direction command Do and current controlled variable I.sub.o for controlling the bridge circuit 44 to the driving circuit 47 as driving signals.
The microcomputer 55 is provided with motor current decision means 56 for generating the rotational direction command D.sub.o of the motor 40 and the motor current command Im corresponding to the auxiliary torque, subtraction means 57 for computing the current deviation .DELTA.I between a motor current command Im and the motor current IM, and PID operation means 58 for computing correction values of P (proportion) term, I (integration) term, and D (differentiation) term from the current deviation .DELTA.I and generating the current controlled variable I.sub.o corresponding to a PWM duty ratio.
Moreover, though not illustrated, the microcomputer 55 includes a publicly-known self-diagnostic function in addition to an A-D converter and PWM timer circuit, detects a trouble in the relay 46 or troubleshoots a system at the start of the system, and unless any trouble is detected, turns on the relay 46 to supply power to the bridge circuit. Furthermore, while the system operates, the microcomputer 55 always self-diagnoses whether the system normally operates. If a trouble occurs, the microcomputer 55 releases the relay 46 through the driving circuit 47 to cut off the motor current IM.
Then, operations of an electric-power-steering controller are described by referring to FIG. 4. The microcomputer 55 captures the steering torque T and vehicle speed V from the torque sensor 50 and speed sensor 51, feedback-inputs the motor current IM from the shunt resistor 43, and generates the rotational direction command D.sub.o of a power steering and the current controlled variable I.sub.o corresponding to the auxiliary torque value to output them to the driving circuit 47.
The driving circuit 47 closes the normally-open relay 46 under a steady driving state. However, when the rotational direction command D.sub.o and current controlled variable I.sub.o are input, the circuit 47 generates a PWM driving signal to apply the signal to the semiconductor switching devices Q1 to Q4 of the bridge circuit 44.
Thereby, the motor current IM is supplied to the motor 40 from the battery 41 through the relay 46, shunt resistor 43, and bridge circuit 44. The motor 40 is driven by the motor current IM to output a required amount of auxiliary torque in a required direction.
In this case, the motor current IM is detected through the shunt resistor 43 and motor-current detection means 48 and returned to the subtraction means 57 in the microcomputer 55 and thereby, controlled so as to coincide with the motor current command Im. Moreover, the motor current IM includes ripple components due to the switching operation of the bridge circuit 44 under PWM driving but it is smoothed and controlled by the large-capacity capacitor 42.
When this type of electric-power-steering controller is started, it performs troubleshooting and thereafter, turns on the relay 46 to supply a control current corresponding to the desired steering torque T to the motor as described above, and operates so as to output a required amount of auxiliary torque. However, because the capacitor 42 has a large capacity, an excessive rush current flows through the relay contact when the relay 46 is turned on. As a result, when the controller is repeatedly started, the contact is welded due to transition and the current supplied to the motor 40 cannot be cut off when a system trouble occurs.
In the case of the relay 46, however, the durability of the contact against the rush current becomes important when the controller is repeatedly started in addition to the fact of satisfying a desired maximum supply current. Thus, a relay having a higher current-supply performance is used as a countermeasure and thus, part costs increase and resultingly, the product cost increases.
In the case of a system requiring a large auxiliary torque, the control current further increases and the impedance must be reduced in order to reduce the heat produced by the capacitor 42 due to increase of a ripple current and thereby, the capacity increases. Therefore, the rush current further increases, and not only the cost increases but also a problem occurs that the reliability of the controller is deteriorated.
The present invention is made to improve the reliability of the controller and reduce the product cost and makes it possible to select a relay in accordance with a maximum control current value by reducing a rush current. Therefore, it is possible to use a relay having a cost lower than a conventional one, reduce the product cost, and improve the reliability.